In recent years, a lightweight low-inertia motor is demanded for various purposes of use.
For example, focusing on a motor for an electric power steering device to be mounted in a vehicle, a higher torque, a lighter weight, and a lower inertia are strongly demanded.
The higher torque is demanded for the purpose of employing the electric power steering device for a large-sized vehicle, the lighter weight is for the purpose of improving fuel efficiency of an automobile, and the lower inertia is for the purpose of improving steering responsiveness.
A high-torque motor has an increased motor body size. Along with this, a weight and an inertia of a rotor also increase.
To cope with this, as a rotor including permanent magnets firmly fixed to an outer circumferential surface of a rotor core, there is known a rotor including lightening portions formed in portions except for an outer circumferential portion that is held in contact with the permanent magnets to reduce the weight and the inertia of the rotor so as to improve the responsiveness (see, for example, Patent Literature 1).